In recent years, significant developments in laser technology have led to its application in the field of ophthalmic surgery. In particular, laser surgery has become the technique of choice for ophthalmic surgical applications. In certain ophthalmic laser procedures, surgeons use a mechanical device termed a microkeratome to cut a layer of the anterior surface of the cornea in order to expose the underlying corneal stroma to which the laser is applied. However, complications surrounding the use of the microkeratome with a metal blade have resulted in research into improved techniques that are performed exclusively by a laser system.
Laser refractive surgeries are performed routinely to treat myopia, hyperopia, astigmatism, and other conditions which cause a patient to rely on vision correction devices (such as contacts and/or glasses) to see. Such procedures include LASIK (Laser Assisted In-Situ Keratomileusis), PRK (Photo Refractive Keratectomy) and LASEK (Laser Subepithelial Keratomileusis) procedures, which use an excimer laser to re-shape a curvature of a patient's cornea through tissue ablation. In many cases, laser refractive surgeries eliminate or greatly reduce the patient's reliance on vision correction devices. Such all-laser techniques provide significant improvements over conventional mechanical devices.
Despite these advances in laser technology, the use of such systems for ophthalmic surgical procedures remains fraught with substantial mechanical limitations, particularly in the area of developing a stable interface between an incident laser beam and the eye of a patient. Ophthalmic surgery is a precision operation and requires precise coupling between the surgical tool (i.e., the laser beam) and the region to be disturbed (i.e., a portion of the patient's eye). Movement of the eye with respect to the intended focal point of the laser beam can lead to non-optimal results and might result in permanent damage to non-renewable tissue within the eye. Given that eye movement is often the result of autonomic reflex, techniques have been developed in an attempt to stabilize the position of a patient's eye with respect to an incident laser beam.
For instance, prior to surgery, the patient's eye is stabilized to prevent translation movement and rotation. In particular, an interface device including a suction ring is placed against sclera tissue of the eye. The suction ring provides a suction pressure to the eye to create a fixation force between the interface device and the eye. After the eye is stabilized, a femtosecond laser is positioned over the interface device and is used to form laser incisions in the corneal tissue to reveal a corneal flap. A surgeon folds over the corneal flap to expose stromal tissue. Next, the surgeon uses the excimer laser to pulse a beam of laser energy onto the exposed stromal tissue. Each pulse removes a very small and precise amount of corneal tissue so that the total removal of tissue alters and corrects the refractive properties of the overall eye. After irrigation with a saline solution, the surgeon folds the corneal flap back in place to adhere to the underlying stromal tissue.
Mechanical stabilization devices have been proposed, for example, a corneal applanation device, which is the subject of U.S. patent application Ser. No. 09/172,819, filed Oct. 15, 1998 (herein incorporated by reference in its entirety), and commonly owned by the assignee of the present invention. Such a mechanical device directly couples a patient's eye to the laser's delivery system being affixed to both the laser and the anterior surface of a patient's cornea. The corneal coupling, in these devices, is typically implemented by lowering an applanation fixture over the anterior surface of the cornea under pressure. Another example stabilization and applanation device is described in U.S. Pat. No. 6,863,667, also herein incorporated by reference in its entirety. It is assumed in these forms of devices that pressure applied normal to the corneal surface will restrict conventional motion of the cornea thereby stabilizing the eye along a major access normal to the device.
However, although this assumption may hold true in a large number of cases, it certainly does not have universal application. Moreover, in the cases where it does hold, the device/cornea interface should be established with the iris centered, for best results. The actual establishment of an effective device/corneal interface is an exercise in trial-and-error, resulting in a great deal of frustration to doctor and patient, as well as considerable eye fatigue.
For ophthalmic laser procedures where eye tissue is to be photodisrupted, it is desirable to have proper focus of the laser beam to a specific focal spot in the tissue that is to be effected. Proper focus includes focal definition and proper dimensionality (i.e., the correct spot diameter and shape). To this end, it is helpful for the laser beam to be as free from aberrations as possible. In particular, for ophthalmic laser procedures involving the cornea, the spherical geometry of the cornea can introduce optical aberrations by its shape, and these are separate and distinct from aberrations that may be introduced by the laser optical system. Corneal induced aberrations can distort the definition of the focal spot of a laser beam as the beam is focused to a position within corneal tissue or deeper into the eye, such as the capsular bag or the natural lens.
Due to the spherical geometry of the anterior surface of the cornea, two specific types of aberrations are of particular importance with regard to beam distortion; spherical aberration (which relates to points on the optical axis of the laser beam) and coma which relates to points that are off-axis). Spherical aberration and coma are similar to one another in that they both arise from a failure to image or focus optical ray traces onto the same point. Spherical aberration relates to a distortion that can be characterized as radial in nature, with some radial directions being stretched while other radial directions are shrunk, converting thereby an ideally circular spot into an elliptical spot. Coma distortion, on the other hand, implies an elongation along one radius a circle, resulting in a “comet-like” shape. Accordingly, any structure which interfaces between a curved, anterior surface of the cornea and laser delivery system will likely encounter such aberration concerns.
In view of the foregoing, it is thus evident that there is a need for a simple mechanical interface device that is able to stabilize the eye against relative motion with respect to a laser beam used for ophthalmic surgical procedures without relying on secondary mechanical considerations, such as surface tension, friction, or the like. Such a device should be able to present an optical feature to an incident laser beam in a stable, well characterized location. In addition to maintaining a proper orientation between the eye and a laser delivery system during ophthalmic laser surgery, such a device should minimize intraocular pressure during the surgical procedure. Such a device should be easy for a clinician to affix, as well as being simple and cost effective to manufacture and use.
Furthermore, although laser refractive surgeries are typically performed without long-lasting side effects, in some instances, the suction pressure applied during stabilization may affect one patient more than another. For example, though the suction pressure delivered to a patient's eye is typically confined within an area defined by an outer diameter of the suction ring, some patients may experience suction pressure outside of the cornea to the conjunctiva. In such cases, the suction pressure may apply a negative relative pressure to the conjunctiva to deform the globe of the eye resulting in ruptured blood vessels, reddening of the conjunctiva post-surgery, and/or an increased intraocular pressure.